The use of two lenses, one plano-convex and one plano-concave, with matching curved surfaces in close proximity to create an adjustable optical wedge or adjustable prism (FIG. 1) to deviate or align a transmitted optical beam has been known since at least 1929 (Cox, U.S. Pat. No. 1,735,108). The beam is deviated by translating or rocking one lens with respect to the other to change the angle between the flat external surfaces (the wedge angle), while keeping the space between the curved surfaces unchanged (FIG. 2). This "ball and socket" type movement is a rotation of one lens about the common center of the two spherical or cylindrical surfaces.
For a small angle (.phi.&lt;10.degree.) wedge used at or near normal incidence in air, the angular deviation (.delta.) of a transmitted light beam is given approximately by, EQU .delta..ident.(n-1).phi. EQ. 1
where (.phi.) is the angle between the two flat exterior faces of the adjustable optical wedge and (n) is the refractive index of the lenses.
The relationship between the translation (T) of the lens in a direction perpendicular to the light beam axis and the angle between the two nominally parallel exterior faces of the device (.phi.) is given by, EQU tan .phi.=T/R EQ. 2
where (R) is the radius of curvature of the curved surfaces.
Throughout this document the term "light" refers not only to visible electromagnetic radiation, but also to ultraviolet or infrared electromagnetic radiation.
Each surface encountered by the light beam reflects a certain fraction of the incident light. The magnitude of this fraction increases as the difference between the two refractive indices increases. This reflection may be eliminated or greatly reduced by the application of an anti-reflection (AR) coating to those surfaces or by making the refractive indices on both sides of the surface equal, as is well known to those skilled in the art.
In many applications, the desired range of deviation of a transmitted light beam is a few degrees on either side of zero deviation (for example -3.degree. to +3.degree.). In these applications it is often desired that the device that deviates the beam direction does not translate the axis of the beam away from its original axis or transverse position. This ability to deviate the direction of a light beam without translating the axis of that beam is a particular advantage of the adjustable optical wedge.
In other applications a similar (.about.6.degree.) range of deviation is desired, but centered about a finite deviation. Nominal deviations greater than zero may be created by adding one or more fixed or adjustable wedges or prisms to the device. Thus, with the lenses in their centered positions, the beam is still deviated. (For example, FIG. 5 shows the optical components of another embodiment of the present invention, an adjustable optical wedge with a range of deviation from .about.27.degree. to .about.33.degree..
If the incident light is polychromatic, the deviation of the device will vary with the wavelength of the transmitted light. This chromatic difference of deviation or dispersion is useful in many application, such as spectrometers. In other applications, such as movie or slide projectors, it is desired to make the deviation the same for at least two different wavelengths. This achromatization may be realized by using two or more different optical materials with differing material dispersions, as is well known in the prior art (FIG. 6).
Larger ranges of deviation may be obtained by using components with sharper curvature (smaller radius of curvature), by using two or more adjustable optical wedges in series, or by combining two adjustable wedges into one. (One example of such a combination is shown in FIG. 7.)
As is well known to those skilled in the art, wedges illuminated through their edge or edges are used to distribute light in numerous applications, such as instrument dials and panels.
Also well known to those skilled in the art is the use of wedges to minimize the aberrations that a tilted parallel plate or window introduces into a converging transmitted light beam.
In some applications the adjustment of the deviation of the beam or beam alignment is made infrequently and the two lenses are kept motionless for extended periods of time. In those cases, the two curved surfaces may be separated while the adjustment is made and then pressed into contact with each other for the extended period during which they are stationary.
In other applications it is desired to adjust the deviation of the beam more frequently. In these cases the curved surfaces can not be left in contact with each other during the movement because they will scratch or abrade each other as they move with respect to each other. This abrasion causes increased scattering of light, increased friction between the two curved surfaces and the creation of objectionable particles. This abrasion problem has previously been mitigated in several different ways, which are reviewed below. None of these prior art devices have been entirely satisfactory.
Merkel, U.S. Pat. No. 3,253,525, taught solving the abrasion problem by placing the two curved surfaces contiguous but spaced. With this approach, a support structure must be provided to keep the lenses separated while one lens is moved with respect to the other. As the movable lens must be rotated about the center of curvature of its curved surface, the structure is often quite large and often blocks the transmitted light beam, as the beam usually passes through that same center. With this approach, both of the curved surfaces and both of the flat surfaces must be antireflection coated if the device is to efficiently transmit a light beam. Also with this approach, the finite gap between the lenses introduces aberrations, distortions, and vignetting of the beam as will be described more fully herein.
These distortions, aberrations and vignettings may be eliminated by reducing the gap to zero thickness or by filling the gap with an index matching material. However, all prior art devices that reduce the gap thickness or fill the gap with index matching material have limitations, as will be described below.
Donelan, U.S. Pat. No. 4,436,260, taught leaving a very narrow gap (less than one wavelength of light) between the two curved surfaces and using an air bearing to keep the surfaces from scratching or abrading each other. A support structure is again required to keep the lenses close to each other and all four lens surfaces must be antireflection coated for high transmission. The air bearing is an expensive and complicated device which subjects the curved surfaces to contaminates that in time degrade the performance of the device. In addition the air bearing requires blocking or distorting part of the clear aperture of the device as a channel must be provided to bring the air into the center of the gap.
Swain, U.S. Pat. No. 4,961,627, taught placing ball bearings between the curved surfaces near the outer rim to keep the separation between the curved surfaces constant and optionally filling the gap with free-flowing refractive index matching liquid. A complicated structure is required to hold and move the lenses in the appropriate manner. A bellows or reservoir structure is required to contain the index matching liquid, which would otherwise flow out of the gap between the two curved surfaces. The index matching liquid eliminates the reflection loss and the need for anti-reflective coatings at the two curved surfaces. The relatively thick layer of index matching liquid may introduce distortions into the transmitted beam due to thermal gradients in the liquid. The motion and reservoir structures are also often large, heavy, and prone to leak.
Harris, U.S. Pat. No. 3,614,194; Cohen, U.S. Pat. No. 4,588,263 and Linder, U.S. Pat. No. 3,884,548 describe adjustable optical wedges with no lubricant and make no mention of a solution to the abrasion problem.
None of these prior art techniques have been entirely successful and adjustable optical wedges have not found widespread use. The present invention is an improved solution to the abrasion problem, which also improves the performance, compactness and cost effectiveness of the adjustable wedge. This improved performance opens up some new scanning and reflective applications for adjustable optical wedges, as will be described more completely herein.